The present invention relates to a method for increasing rounding effect in a trench top corner, and more particularly to a method for pulling back SiN to increase rounding effect in a shallow trench isolation process (STI) and to avoid Wrap Round in the trench top corner.
Trench isolation is a widely used method in the modern VLSI process to isolate elements. The primary principle of the technique is to use the anisotropic dry etch to define a trench and then a filling is added to the trench. As the trend of the modern semiconductor, the dimension of the elements used in the semiconductor is smaller and smaller, which causes the dimension of the isolation layer to decrease. Accordingly, semiconductors having the structure of STI become the mainstream.
With reference to FIGS. 1A to 1H, the conventional STI technique includes the following steps. First, a SiO2 layer 102 is formed on a substrate 101 of Si. A Si3N4 layer 103 is deposited on the SiO2 layer 102 to form a shallow trench area 104. Then, high density plasma chemical vapor deposition is used to precipitate an oxide 105 in the shallow trench area 104. After the precipitation step, it is necessary to planarize the oxide 105. A step of rounding the shallow trench corner 104a is then processed. After the rounding process, the Si3N4 layer 103 is removed. Finally, cleaning steps including wet cleaning, cell cleaning and tunnel cleaning are performed.
In the conventional method, the oxide 105 is planarized in an etcher and then cleaned. After the post cleaning process, an oxide recessed portion 106 is often formed on the edge of the oxide 105 in the shallow trench area 104. The recessed portion 106 causes a wafer to have abnormal conductivity, such as double humps in the Id-VG curve.
Therefore, it is necessary to provide an improved method to form a shallow trench isolation structure to eliminate the wrap around in the trench top corner.
To overcome the aforementioned problem, some introduces a method, as shown in the prior art and in FIGS. 2A to 2I, to protect the edges of the wafer, which includes the following steps. The first step is to prepare a SiO2 layer 202 and a Si3N4 layer 203 on a substrate 201 of Si. Then, a photoresist layer 204 that defines an opening 205 is formed above the Si3N4 layer 203. An anisotropic etching is applied to the SiO2 layer 202 and the Si3N4 layer 203 to form an encasing wall 206a around the photoresist layer 204, the SiO2 layer 202, the Si3N4 layer 203 and the opening 205. Thereafter, a dry etching is introduced to the encasing wall 206b that encloses the opening 205 and the substrate 201 to form a shallow trench area 207. Then, the photoresist layer 204 and the encasing wall 206b are removed to expose the unetched area on the substrate 201 and a sharp edge 208a. After the removal of the photoresist layer, it is then required to form an oxide of Si 209 on the unetched area of the substrate 201 and the sharp edge 208a to change the sharp edge 208a to round edge 208b. Then it is necessary to precipitate an insulation layer 210 on the oxide of Si 209 and fill the shallow trench area 207. Last, the SiO2 layer 202 and the Si3N4 layer 203 are removed from the insulation layer.
This method uses the encasing wall 206b of a polymer to fill in the opening 205. When the polymer is removed and the insulation layer 210 is filled in the space left by the removal of the polymer, the insulation layer 210 is able to protect the corner 211.
This method does meet the necessary requirements. However, it needs the step of forming the encasing wall and the removal of the encasing wall, it is too complicated and the cost of manufacturing is increased.
According to the foregoing technique, the methods such as wet etch or oxidation to pull back the SiN complicates the process and increases the cost. Furthermore, after the Si3N4 layer is removed, the post cleaning process easily forms wrap round on the trench top corner and thus causes high electric field and pre-breakdown.
To overcome the shortcomings, the present invention intends to provide an improved method for pulling back SiN to increase rounding effect in a shallow trench isolation process and to avoid Wrap Round in the trench top corner.
The primary objective of the present invention is to provide a method for pulling back SiN to increase rounding effect in a shallow trench isolation process and to avoid Wrap Round in the trench top corner.
To order to accomplish the foregoing objective, the method adds an isotropic etching process to pull back the Si3N4 and to increase the trench top corner rounding. After the SiN layer is etched to a predetermined depth, a gas of SF6/HBr is applied to fully etch the remaining SiN. With the gradient variation of the depth by the SF6/HBr, and the etch rate selectivity of SiO2 to Si being less than 1, a top rounding etch step is adopted to continue etching the oxide layer and the substrate to have a reinforced top corner rounding.
Other objects, advantage and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.